First of all the PowerMacs are the Mac desktop computers running on G5 PowerPC processors.
From my experience FCP 5.1 will open a project from an earlier version except maybe if you used third party plugins not compatible with 5.1.
You definitly cannot open a 5.1 project in 5.0x.
If you open a 5.0x project in 5.1 it will convert it to a 5.1 project therefore you can’t open it back in 5.0x.
Suggestion install 5.1 at home.